Liquid-fuel-fired burners capable of producing an ignitable mixture of the fuel and combustion air and of sustaining such combustion in the fire box or combustion chamber of a furnace, boiler or the like, are known. Such systems have a feed line for the liquid fuel, a duct through which the combustion air is supplied, and even an arrangement whereby the hot combustion or exhaust gases from the combustion chamber and before discharge to a stack, chimney or the like, serves to preheat the combustion air.
Burners of this type can be used in industrial furnaces as well as in small heat-generating plants and are advantageous because the recovery of heat from the waste gases and the preheating of the combustion air reduces the fuel requirements.
When the fuel is available in a liquid state, e.g. is fuel oil or heating oil, it is important to ensure good mixing of the fuel and the air so that the combustible mixture is more or less homogeneous and uniform and can sustain a uniform combustion throughout the region into which the combustible mixture is discharged.
In earlier systems utilizing liquid fuels, the fuel is sprayed, atomized or otherwise dispersed into the preheated combustion air which can be at a temperature sufficient to rapidly evaporate the fuel, the evaporation rate being greater as the fuel surface area increases with a finer subdivision of the liquids.
Reference can be made to the dissertation by R. Buchheim, RWTH-Aachen, Germany, "Untersuchungen zum Emissionsverhalten von Pkw-Gasturbinenbrennkammern", 1977.
Thus the generation of an ignitable mixture in accordance with these principles requires that the air be at a temperature above the boiling point of the liquid fuel.
However, with air preheated to such temperatures cracking processes can occur in the fuel which can result in the deposition of carbon black, soot or other particulates in the combustion chamber and in ducts or passages traversed by the exhaust gas.
Consequently, the application of the principle has been limited or had to be combined with relatively frequent cleaning of the burner system and/or the combustion chamber.